Here's a way to have a rear-view mirror on your bike, even if you rent one.

RearViz is, I assume, short for 'rear vision'. This folding and fully rotatable mirror allows for adjustment to find perfect angle of vision. The company calls it a slim fit housing, and I guess it is, considering the fact there's a fairly wide-angle mirror inside. The wide elastic arm band fits any arm size. The mirror can be worn on the wrist or on the arm. Since this mirror is designed to be worn on the body instead of on the bike, that should help eliminate road vibrations while letting you see what's coming up from behind you. Great for those who rent bikes too, like Citibike. You can always have a rear view mirror when you bike. Beware, you most likely will also see a bit of your arm and the device itself in the view because, well, it's on your arm. You don't need to get it in pink. Other colors are available. It was on Amazon, but now it shows 'unavailable'.

Here are other Amazon choices: https://amzn.to/2SSESOi

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Here's the company website: https://www.rviactive.com/collections/all